1. Field of the Invention The present invention relates to a spread spectrum communication system for the mobile communication, wireless LAN etc.
2. Description of Related Art
The spread spectrum communication system seems hopeful in the fields of mobile cellular radio and wireless LAN due to its frequency efficiency and confidentiality.
The modulation and demodulation of a direct-sequence code division multiple access (DS-CDMA) type spread spectrum communication are as follows.
On the transmitter side, the modulated signal consists of an in-phase component (I-component) Si and a quadrature component (Q-component) Sq. EQU Si.dbd.IiCi-IqCq (1) EQU Sq.dbd.IiCq+IqCi (2)
Here, information to be transmitted is divided into an I-component Ii and Q-component. Ci is a spreading code of an I-component and a Cq is a spreading code of a Q-component.
On the receiver side, the demodulated signal consists of an I-component Ri and a Q-component Rq. EQU Ri.dbd.IiCi-IqCq (3) EQU Rq.dbd.IiCq-IqCi (4) EQU Di.dbd.RiCi+RqCq (5) EQU Dq.dbd.-RiCq+RqCi (6)
Then, EQU Di.dbd.Ii(Ci.sup.2 +Cq.sup.2) (7) EQU Dq.dbd.Iq(Ci.sup.2 +Cq.sup.2) (8)
Here, the received signal is despreaded and divided into the I- and Q-components Di and Dq.
FIG. 30 shows a conventional receiver including a matched filter of the CDMA communication system. An analog signal Ain4 of an intermediate frequency (IF), received by the receiver, is divided into the two signal components Ri and Rq. The carrier signal is removed by a low-pass filter LPF, then, Rq is input to matched filters MF1 and MF2 and Ri is input to matched filters MF3 and MF4. The spreading code Cq for Q-component is given from a code generator PNQ to MF2 and MF3, and spreading code Ci for I-component is given from a code generator PNI to MF1 and MF4. Each matched filter multiplies the signal component of successive time sequences by the spreading code and sums them up.
FIG. 31 shows a conventional receiver including a sliding correlator, which converts the received signal Ain5 of the IF into I- and Q-components by the quadratic detector and LPF. The I- and Q-components are multiplied by Cq and Ci by four multipliers shown by X. Then, the I- and Q-components are composed by the adders similarly to the above. The result of the composition is coherently tracked and a correlation peak is detected.
In such a communication system of a plurality of components divided from the signal to be transmitted, the more the number of components becomes, the higher the communication speed is. The communication capacity becomes better. The communication reliability is improved in view of error ratic etc. However, the communication apparatus becomes more expensive and the electrical power consumption increases.
FIG. 32(A) shows a frame format of transmission data of DS-CDMA communication. Each frame consists of one pilot symbol block P1, P2, . . . , Pn and one information symbol block I1, I2, . . . , In, respectively. Each pilot block has predetermined L symbols, or pits L can be, for example, "4". Each information block has predetermined number, for example "36", of symbols or bits. These symbols are modulated by QPSK and spreaded by the predetermined spreading code sequence.
At the receiver, a fading complex envelope is estimated from the pilot symbols and fading compensation is performed by using the estimated envelope. The coherent detection and rake combining are performed. A frame synchronization signal is output when the pilot symbol block is of a predetermined pattern.
FIG. 32(B) shows a conventional rake receiver. Each I- and Q-component of an output of a complex matched filter 80 is input to a delay profile detection circuit 82, a signal level detection circuit 84 and a phase correction circuit 86. The first path of the multi-path is detected by the circuit 82 which outputs a signal to a frame synchronization circuit 83 for detecting a timing of the frame synchronization. The timing is input to a phase correction portion 86. A rake combiner 87 and a data decision portion 88 are serially connected to the phase correction portion 86.
The phase correction portion calculates a phase rotation of the pilot symbol by comparing the received pilot symbol with a predetermined pilot symbol. Then, a correction vector is calculated for compensating the phase error due to fading etc.
In the spread spectrum communication system for wireless local area network (LAN) etc., the information is differentially encoded at a transmitter. At the receiver, the delay detection is performed according only to the strongest path among multi-path, so the accuracy of the frame synchronization detection is rather low. The reliability of the received data is low.